The present invention relates to latching mechanisms for the doors of appliances such as clothes washing machines and, in particular, to an electrically actuated lock for such latching mechanisms.
Appliances, such as clothes washing machines and dishwashers, may operate automatically through one or more cycles under the control of a timer. During cycles when the consumer might be exposed to spraying water or hazardous moving parts, the door to the appliance may be locked by an electrical signal from the timer. The locking mechanism may, for example, insert a blocking member into a portion of the door latch to prevent the latch from opening, or the locking mechanism may insert a bolt directly between the appliance frame and door.
One method of actuating the locking mechanism is to use a thermal actuator, for example, a wax motor or bi-metallic strip. Such thermal actuators have the disadvantage of requiring a heating or cooling of a material. This heating or cooling process typically takes some time, preventing rapid locking or unlocking of the locking mechanism.
An alternative method of actuating the locking mechanism is to use an electrical solenoid having a ferromagnetic armature that moves through a conductive coil when electrical power is applied to the coil to form an electromagnet. Electrical solenoids provide for rapid actuation but at a cost of increased size and expense, particularly if the coil windings, the latter of which normally must be rated for continuous duty to maintain the locking mechanism in its locked or unlocked state. The use of continuous duty solenoids in locks can also create a problem in the event of a electrical power loss, caused either by an electrical outage, or the appliance being unplugged, where the end user will have access to the inside of the washer while the drum is still spinning. The disadvantages inherent in the use of a continuous duty solenoid can be overcome through the use of an electromagnetically pulsed bi-stable solenoid. A bi-stable solenoid may include a magnetized armature movable in different directions by different polarities of current through a single solenoid coil, a pair of back to back independent solenoid coils passing a ferromagnetic armature between them when one or the other is energized, or a single solenoid activating a mechanism that cycles between two states with each activation. The bi-stable solenoid may be coupled with an over-center spring or the like to hold the armature in its last position when no power is applied or during a power loss.
Such bi-stable solenoids provide rapid actuation and overcome the power dissipation problems inherent with continuous duty solenoids. When used in a locking application, however, they have an important shortcoming. In the event of a loss of electrical power, the door latch may be locked indefinitely because no power is available to move the bi-stable solenoid to its unlocked state. This is an important problem in commercial laundry establishments where, in the event of power failure, customers will not be able to collect their clothes and yet may be reluctant to leave their clothes unattended.
U.S. Pat. No. 6,363,755 to Hapke et al issued Apr. 2, 2002 and entitled “Timed Release Washing Machine Lid Lock” describes a circuit that holds energy in a capacitor to be used to unlock a bi-stable lock mechanism at the end of a spin cycle even if power has been lost. This approach, while extremely flexible, requires an additional electrical circuit and a capacitor sized to store sufficient energy, which can be expensive.
U.S. Pat. No. 5,572,869 to Schantz et al issued Nov. 12, 1996 and entitled “Actuator Assembly for use in Appliances” describes a “wax motor” that uses an internal heating element to generate a force and displacement. This device stores energy within a compression spring that can be used to deliver a secondary force and displacement (in the opposite direction) after electrical power is removed and upon cooling of the heating element (which occurs after some time delay).